Heretofore, as typically illustrated by schematic vertical sectional views in FIGS. 4(a) and (b), it is known that high-purity silica glass powder having an average particle size of 200-300 μm and a purity of not lower than 99.99% is used as a starting material and filled into a space defined by an inner surface of a graphite mold and an outer surface of a core, for example, a space of 30 mm while rotating the graphite mold at a speed of 60-80 rpm [see FIG. 4(a)], and then the core is taken out after the filling, and a three-phase AC arc discharge apparatus using graphite electrodes is inserted from an upper opening part while rotating the graphite mold at a speed of 50-100 rpm and reciprocated vertically along the inner surface of the graphite mold to heat an inside of the graphite mold to a temperature of about 2000° C., while the starting powder is melted and solidified while conducting vacuuming through an air flow path opened to the inner surface of the graphite mold, whereby a silica crucible having, for example, a thickness of 10 mm is produced [see FIG. 4(b)].
Also, it is known that the resulting silica crucible has a two-layer laminated structure composed of an outer layer of high-purity amorphous silica glass having a bubble content (expressed by a percentage of a total volume of bubbles contained in silica glass per unit volume) of 1 to 10% and a purity of not lower than 99.99% and an inner layer of high-purity amorphous silica glass having a bubble content of not higher than 0.6% and a purity of not lower than 99.99% and a thickness ratio of the inner layer to the outer layer is commonly adjusted so as to be 1:1-5 [see FIG. 6].
Furthermore, as shown by a schematic vertical sectional view in FIG. 5, it is well known that the single crystal ingot is produced by charging a high-purity polycrystalline silicon lump into a silica crucible mounted onto a graphite support, melting the polycrystalline silicon lump by a heater disposed along an outer periphery of the graphite support to form a silicon melt, and heating and maintaining a temperature of the silicon melt to a given temperature range of 1500-1600° C. and bringing a silicon seed crystal into contact with the silicon melt surface while rotating the silica crucible and simultaneously rotating the silicon seed crystal in an Ar gas atmosphere under a reduced pressure to pull it.
Moreover, in the above production of the single crystal ingot, as also shown in FIG. 5, the silicon melt moves through convection flow of flowing downward in the silica crucible from the bottom of the single crystal ingot to the bottom of the crucible and then flowing upward along the inner surface of the crucible from the bottom thereof toward the bottom of the single crystal ingot, during which the silicon melt (Si) and the inner surface (SiO2) of the crucible are reacted to generate SiO gas. The thus generated SiO gas moves toward the silicon melt surface together with the flow of the silicon melt and is released and removed into the Ar gas atmosphere under a reduced pressure, but it is incorporated into the single crystal ingot during the pulling to cause pinhole defect, so that the ingot is produced by adjusting the pulling conditions so as not to cause such a defect.
In addition, for the purpose of preventing deformation and distortion of the silica crucible during the pulling of the single crystal ingot to prolong the service life of the silica crucible, it is also attempted to improve the strength of the silica crucible by interposing a crystallization promoter made of an oxide, a hydroxide, a carbonate or the like of an alkali earth metal between the inner layer and the outer layer of the silica crucible, or by applying the crystallization promoter onto an upper opening portion of the crucible along an outer peripheral surface thereof to transform an amorphous structure into a crystalline structure by the action of the crystallization promoter during melt-molding.
Patent Document 1: JP-B-H07-42193
Patent Document 2: JP-A-2000-169287
Patent Document 3: JP-A-H11-171687
Patent Document 4: JP-A-2006-124235
Patent Document 5: JP-A-2005-255488